The invention relates to a method of testing a borehole in an underground formation by the use of so-called closed chamber testing, wherein a test pipe is lowered into the borehole, which pipe is closable at its upper end and at its lower end is provided with a downhole assembly comprising equipment for testing of fluid flow from the formation, the annulus between the test pipe and a casing in the borehole being shut off during the test by a gasket at a desired depth, and fluid from the formation being allowed to flow through the test pipe to a collecting tank coupled to the test pipe at the upper end thereof.
Further, the invention relates to a system for such testing, comprising a test pipe which is adapted to be lowered into the borehole and at its lower end is provided with a downhole assembly comprising equipment for testing of fluid flow from the formation, a gasket for shutting off the annulus between the test pipe and a casing in the borehole, and a collecting tank coupled to the test pipe via a flow head at the upper end of the test pipe.
As will be known to a person skilled in the art, testing of petroleum wells are carried out to find out the petroleum production potential of the well and to measure the properties, characteristic and spreading of the reservoir and the reservoir fluid. In such testing, different testing methods are used, including so-called closed chamber testing. The existing methods of this type typically utilize an empty chamber (filled with air or nitrogen), which produces a high differential pressure over the reservoir surface. This results in a shock wave with high velocity, which is intended to remove possible debris or possible blockings from the perforation tunnels, but may also result in formation brakedown. The inflow velocity at the beginning will be high, but will decrease as the chamber is filled with a heavier fluid.
The known systems have a number of weaknesses which can be summarized as follows:
mixing of borehole and reservoir-fluids,
lack of accurate flow velocity measurements and volume control,
lacking ability of achieving representatives specimens of borehole fluids because of contamination,
constantly varying flow velocities, the chamber typically running xe2x80x9cemptyxe2x80x9d, so that an initial shock wave will arise, followed by gradually lower velocities as the chamber is filled,
high probability of slug flow (irregular two-phase flow) from zones having a low productivity because of gas breakouts,
no real time downhole data
interpretation of transient data because of varying flow velocity and storage effects,
not suitable for testing of wells having a high flow potensial.
On this background it is a general object of the invention to provide a method and a system, based on closed chamber testing, wherein the above-mentioned weaknesses are at least essentially eliminated.
A more specific object of the invention is to provide a method and a system wherein the flow velocity of the formation fluid can be measured accurately by controlling the inflow and thereby the downhole pressure.
Further objects of the invention are to provide a system which facilitates testing and sampling without producing well fluids to the surface, and wherein the system is constructed such that a test can be stopped at any time and fluids reinjected into the reservoir.
For achieving the above-mentioned objects there is provided a method of the introductorily stated type which, according to the invention, is characterized in that, in a pipe section at the lower end of the test pipe, there is releasably retained a pig forming a barrier between formation fluid and a lightweight damping fluid filling the pipe above the pig, the pig being released at the start of the test and being moved in a controlled manner upwards in the pipe as a result of a positive pressure difference between the fluids below and above the pig.
Further, there is provided a system of the introductorily stated type which, according to the invention, is characterized in that it comprises a pig arranged to be retained releasably and in a sealing manner in a pipe section at the lower end of the test pipe, and a reservoir for a lightweight damping fluid arranged to be supplied to the test pipe via the flow head, in order to substantially fill the test pipe above the pig at the start of the test, so that the pig forms a barrier between fluid from the formation and the damping fluid above the pig.